1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition suited, for example, for ultrasonic oscillators, actuators, piezoelectric transformers, or the like, and to a piezoelectric transformer used in electric power conversion devices such as an inverter for the back light of a liquid crystal display.
2. Description of the Prior Art
Piezoelectric materials are applied to various devices, such as ultrasonic oscillators, actuators, and piezoelectric transformers, as materials which can convert electrical energy to mechanical oscillation energy or mechanical oscillation energy to electrical energy. In these applications, piezoelectric materials having a large electromechanical coupling factor and a large mechanical quality factor are used. An example of such piezoelectric materials is a composition comprising Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 --Pb(Sn.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 to which MnO.sub.2 is added, which has been known to have an excellent piezoelectric properties (see Japanese Patent Publication No. 56-30714).
In recent years, increase in the output of piezoelectric devices is demanded. However, the mechanical quality factor of piezoelectric materials lowers non-linearly due to high-field driving and increase in mechanical vibration, and piezoelectric materials abruptly generate large heat due to the loss of internal energy. Therefore, piezoelectric materials having a mechanical quality factor which lowers little even at high-field driving have been demanded.
With recent downsizing of office automation facilities, piezoelectric transformers have been diffused for producing inverters for cold cathode-ray tubes used as the back light of a liquid display, and for other applications. A piezoelectric transformer converts inputted electrical energy to mechanical oscillation energy, then to electrical energy again, thereby steps up or steps down the input voltage. For piezoelectric transformers used in such applications, high conversion efficiency and a small and thin size are demanded.
Also, a piezoelectric transformer is a device utilizing the resonance of a piezoelectric element, and when oscillation in the length direction is used, the resonant frequency increases with decrease in the length of the element. That is, downsizing leads to increase in driving frequencies. However, if driving frequencies are increased, the conversion efficiency of the entire circuit containing a piezoelectric transformer, such as an inverter circuit, is lowered by the effect of floating capacity in the vicinity of the circuit, the frequency characteristics of the cold cathode-ray tube, or the like. Therefore, small piezoelectric transformers which can be driven by a relatively low frequency are demanded.